A liquid crystal display (LCD) which is a display device formed of a display element such as a liquid crystal is widely used. On the other hand, in recent years, a so-called self-light emitting display device, that is, a light-emitting device has been attracting attention, which has a pixel formed of a display element such as a light-emitting diode (LED). As a display element used for such a self-light emitting display device, an organic light-emitting diode (also referred to as OLED (Organic Light-Emitting Diode), an organic Light emitting element, electroluminescence (EL) element, or the like) has been attracting attention, and used for an EL display and the like. A display element such as an OLED is self-light emitting; therefore, it has advantages such as higher visibility of pixels, no backlight, and higher response speed compared to a liquid crystal display.
These days, an EL display has been particularly used in a display portion of a portable electrical appliance such as a mobile phone. In such a portable electrical appliance, still lower power consumption is required.    [Patent Document 1] Japanese Patent Laid-Open No. 2002-297106